Like all things, you start with an idea for what to do. My suggestion would be that you then work out specific details for the various pieces; sort of like a 'storyboard' for a movie.
Where you go from there depends a lot on the complexity of the game development. If you are replicating a board game, like tic-tac-toe, that's one level of effort. If it is rich in graphics and sound, that adds more levels of effort. A FPS like Quake is even more effort. All that takes planning and resources.
get out the pen and paper and PLAN it. trust me it saves a lot of work. Make sure everyone is doing sonething to help ie someone making data (drawing stuff, making sounds ect...) someone is coding the menu / user gui and someone coding the game system. there are a few libraries out there to get you started for example there is OpenGL and DirectX for graphics
Its tons of work, there are tutorials out there that can help you decide. Please refer to google for them they are every were. They help every one, most of my friends are game programmers. They use these tutorials and there games come out great.